Microbial biomass measured as total lipid phosphate in soils of different organic content

Abstract

The use of total lipid phosphate as a measure of biomass was evaluated in soils with different organic matter content. Lipids were extracted with a one-phase mixture of chloroform, methanol, and a buffer, and digested with either persulfate or perchloric acid to liberate lipid-bound phosphate. This procedure was evaluated by varying the extraction buffer, the extraction and digestion times, the amount of soil extracted, and the amount of lipid material digested. An extraction period of 2 h was sufficient to yield maximum lipid phosphate. Neither sonication, vigorous mixing, nor longer extraction periods increased the amount of lipid phosphate extracted. However, the amount of lipid phosphate recovered was dependent on the choice of buffer. When organic soil was used, citrate buffer in the extraction mixture gave higher amounts of lipid phosphate than acetate buffer, Tris, H2O or phosphate buffer. In a sandy loam with low organic matter content, citrate or phosphate buffers performed equally well. When 13 soils of different organic matter content were examined, the two digestion methods showed a good linear correlation (r2 = 0.991). Substrate-induced respiration (SIR) and ATP contents of the different soils correlated well with the total lipid phosphate. Based on these measurements, a conversion factor of 500 μmol lipid phosphate·g-1 biomass-C (perchloric acid digestion) was calculated. © 1991.